Closed circuit resistive shielding for multiwinding transformers



Dec. 13, 1966 E. A. DORMAIER 3,292,127

CLOSED CIRCUIT RESISTIVE SHIELDING FOR MULTIWINDING TRANSFORMERS Filed July 1, 1963 INVENTOR.

ELMER A. DORMAIE ATTORNF'V United States Patent Ofiice 3,292,127 Patented Dec. 13, 1966 3,292,127 CLOSED CIRCUIT RESISTIVE SHIELDING FOR MULTIWINDING TRANSFORMERS Elmerv A. Dormaier, Orange, Calif., assignor to Beckman Instruments, Inc., a corporation of California Filed July 1, 1963, Ser. No. 291,772 7 Claims. (Cl. 336-84) The present invention relates to shielding of electrical components and more particularly to shielding of transformer windings.

Transformers which are employed in high accuracy electrical circuits generally include one or more electrostatic shields. An internal electrostatic shield is employed to prevent capacitive coupling between the windings of the transformer. These shields are ordinarily constructed of a conductive member such as copper, with a continuous conductive member providing the most effective shield. However, the electromagnetic induction between the transformer windings must not be adversely affected by the shield, and therefore, the shield cannot constitute a short-circuited turn around a transformer winding. It is also desirable that the shield have a low inductance. It becomes diffic-ult to provide one or more shields of the desired electrical characteristics when the size and shape of the over-all transformer must be maintained within a given physical size.

When shielding transformers with a continuous conductive member, such as copper foil, normally it is necessary to cut and fit the shielding by hand. In constructing a transformer, a winding is placed on a core, followed by suitable insulation which may be tape or the like followed by the shield. It is necessary to provide complete coverage with the shield. Since ashort-circuited t-urn cannot be tolerated the ends of the shield must be overlapped with a gap being provided in the shield by interposing an insulator or the like. This further increases the difficulty of constructing transformers with one or more shields. Additionally, different size transformers employ different size cores and windings, and therefore, the ,shields must be of different sizes. Where multiple shields are desired, it becomes increasingly difficult to maintain the overall dimensions of the transformer within reasonable limits.

In the past, it has been proposed to provide shields by using a silver paint. The use of silver paint essentially solves the problem of maintaining the external dimensions of the transformer within reasonable limits. However, such paints are highly conductive and hence a gap in the shield is still required in order to prevent a shortcircuited turn. The silver paint is painted on an insulating coating which is applied over the winding. It has been found, that the silver or the silver paint frequently tends to migrate into the insulating coating over a period of time thereby decreasing the insulation resistance provided by the insulating coating.

Accordingly, it is a feature of the present invention to provide improved shielding for transformer windings without the inherent difficulties of construction and drawbacks of prior shields.

An additional feature of the present invention is the provision of a transformer shield which includes a resistive paint whereby no gap in the shield is necessary.

A further feature of the present invention resides in providing a plurality of shields comprising resistive paint "between the windings on a transformer.

ping of tape or other suitable material for providing an The layer may be coated with a suitinsulation layer.

able cement to prevent seepage of a subsequently applied layer of resistive paint which provides a shield. The resistive paint has a sufficient resistive value such that it does not constitute a short-circuited turn, and can have a sufficiently low resistive value in order to still maintain sufficient shielding properties. The resistive paint may be applied by brushing, spraying or dipping. The resistive paint is followed with another insulating layer upon which a second transformer winding may be wound. If desired, plural shields may be provided by applying the insulating layer, resistive paint, insulating layer, resistive paint, etc.

Other features and objects of the present invention will become apparent through a consideration of the following detailed description taken in connection with the accompanying drawing in which:

FIG. 1a is a perspective view of a partially completed transformer which is shielded in accordance with the present invention;

FIG. 1b is a cross-sectional view of FIG. la taken along the line BB; and

FIG. 2 is a cross-sectional view of a plural Shield transformer constructed in accordance with the teachings of the present invention.

Referring now to FIG. 1a, a broken sectional View of a partially completed transformer is shown. The transformer includes a laminated core 10 encased in an insulating envelope 11. It is not necessary that the core 10 be laminated, but it may be a moled core made of a conventional material, such as powdered iron. The envelope 11 may be made of a material such a nylon. A conductor is wound about the envelope 11 to provide a winding 12 which may constitute either the primary or the secondary of the finished transformer. Tape 13 is wound about the winding 12 to act as an insulator between the winding 12 and a first shield. The tape 13 may be, for example, .001 inch ungummed Mylar tape. Although not shown in FIG. la, the tape 13 preferably is coated with a cement to prevent seepage of a subsequently applied resistive shield coating between the layers of the tape 13. Typically, the Mylar tape 13 is applied with a 50% overlap an'd brushed with a coating of epoxy cement, such as TC-438 or 3047 epoxy cement manufactured by Electronic Production and Development, Hawthorne, California. The epoxy cement may be cured for a few hours at room temperature, or cured in a shorter time by moderately heating. If the cement is not applied, resistive paint seepage may tend to lower the insulation resistance provided by the tape 13. Since the inside diameter of the toroidal core is smaller than the outside diameter thereof, the tape 13 builds up a greater thickness inside. Hence, it frequently is desirable to lay a piece of tape about the outer periphery of the tape 13 in order to provide substantially equal insulation resistance on both the inside and outside layers.

According to a feature of the present invention, a

shield 14 of a resistive coating is applied over the cemented tape 13. The resistive coating may applied by brushing, spraying or dipping. Various types of resistive coatings are available, with the R-series resistance micropaints manufactured by Micro-Circuits Company, New Buffalo, Michigan, being found satisfactory. Particularly suitable types are Micro-Circuits type RS17B and R814. FIG. 1b is a cross-sectional view of FIG. la taken along the lines BB, and further illustrates the manner in which the various layers are juxtaposed.

Although not shown in FIG. 1a, a tape and cement layer is applied over the resistive shield 14 followed by a second winding. The second winding may be suitably insulated with a final wrapping of tape.

FIG.2 is a cross-sectional view (similar to FIG. lb) of part of a transformer having a plurality of shields convalues.

structed in accordance with the teachings of the present invention. Equivalent parts in FIGS. 1a, lb and 2 are indicated by like reference numerals. Three shields are provided on the transformer shown in FIG. 2. An insulating envelope 11 is provided around the core 10. The winding 12 is wound around the envelope 11, followed by tape and cement 13 and a resistive shield 14. An insulator 15 of tape and cement or the like is provided over the resistive shield 14, and is followed by a second resistive shield 16. In a similar manner, an insulator 17 is provided over the resistive shield 16, and followed by a third resistive shield 18. Another insulator 19 is provided around the resistive shield 18 and a second winding 20 is wound over the insulator 19. A final layer of tape 21 is provided over the winding 20, and this tape layer may be employed with or without a cement coating as desired. Suitable leads may be connected to the windings 12 and 20. Leads may be connected to the shields by taping part of a lead to the tape insulator and applying the resistive coating to the tape and the lead. This provides a suitable electrical connection between the coating and lead.

Either the winding 12 or the winding 20 may serve as the primary winding or the secondary winding of the transformer. For example, when the transformer is used in the input stage of a floating input to grounded output circuit, the primary winding may bethe winding 12; whereas, when the transformer is used as in the output circuit, the secondary winding may be the winding 12. Reference may be made to US. patent application Serial No. 151,604, filed November 13, 1961, entitled Wide- Band Amplifier and assigned to the assignee of the present invention for an example of a high accuracy amplifier in which shielded transformers constructed in accordance with the present invention may be utilized. Typically, the transformer shields are connected to appropriate points within the amplifier or associated electrical circuitry.

Depending upon the desired resistance of the resistive shield, one or more coats of the resistive paint may be utilized. For example, one brushed coat of the type RS14 paint or two brushed coats of the RS17B paint have been found suitable. Although the resistance of the shield depends upon the thickness of the coating, the thickness has not been found to be critical. The resistivity of the coating is stated in terms of ohms per square. The type R814 paint provides around ohms per square for a fairly heavy coat, and 2 to 3 ohms per square for a very heavy coat; whereas, a typical silver paint has a conductivity of approximately 0.5 ohm per square. If the resistive paint is thinned, a higher resistance results, but several coats may be applied to give lower resistive The type RS17B resistive paint typically has a resistance in the range of 50 to 200 ohms per square depending on thickness. The effective resistance of the shield depends on the resistance per square of the coating,

thickness and the size of the area coated. These resistive values are given as merely exemplary and it is to be understood that the actual resistance of the shield may be higher or lower depending upon transformer size (related to core size), thickness of paint, etc., and upon the particular application for the transformer. The primary requirements are that the equivalent resistance is not so low that the shield constitutes a short-circuited turn and overloads the transformer, and that the resistance of vthe shield is not so high that shielding efiiciency is impaired.

It now should be apparent that the present invention provides an improved shield and method of providing a shield for a transformer. Although exemplary embodiand arrangements are possible and the the embodiments disclosed may be subjected to variouschanges, modifications and substitutions without necessarily departing from the spirit of the invention.

What is claimed is:

1. In a transformer having at least a primary winding and a secondary winding, and insulation on each of said windings, the improvement comprising a shield between said windings and completely enveloping one of said windings, said shield being constructed of a closed circuit resistive coating, said coating a having a sufiicient resistive value to avoid the effects of a short-circuited turn overloading said transformer, and to avoid a resistance so high as to impair the shielding effect of said shield.

2. In a transformer having a plurality of windings and at least one shield interposed between said windings, the improvement comprising said shield comprising a closed circuit resistive coating which completely encompasses one of said windings thereby shielding said windings from each other, said coating having a sufficient resistive value to avoid the effects of a short-circuited turn overloading said transformer, and to avoid a resistance so high as to impair the shielding effect of said shield.

3. A device as in claim 2 wherein said transformer includes a plurality of shields each of which is composed of a closed circuit resistive coating, with a first shield over an insulator located around a first of said plurality of windings, and with a second shield around an insulator which is located over said first shield, said shields each having a suf ficient resistive value to avoid the effects of a shortcircuited turn overloading said transformer, and to avoid a resistance so high as to impair the shielding effect of said shields.

4. In a toroidal transformer having a first winding on a toroidal core and an insulator around said first winding, the improvement comprising a shield including a closed circuit resistive coating completely enclosing said insulator, said coating having a suflicient resistive value to avoid the effects of a short-circuited turn overloading said transformer, and to avoid a resistance so high as to impair the shielding effect of said shield,

a second insulator around said shield, and

a second winding around said second insulator.

5. In a transformer having a core, a first conductor wound on said core, an insulating tape wound on said conductor and a cement coating on said tape, the improvement comprising a shield which completely encompasses said tape and said cement coating, said shield being constructed of a closed circuit resistive coating, said coating having a sufficient resistive value to avoid the effects of a short-circuited turn overloading said transformer, and to avoid a resistance so high as to impair the shielding effect of said shield,

at second layer of tape wound about said shield, and

a second conductor wound about said second layer of tape.

6. In a transformer having at least first and second windings, an insulator on said first winding, the improvement comprising a first shield encompassing said insulator, said shield being composed of a closed circuit resistive coating,

a second insulator on said shield,

a second shield encompassing said second insulator,

said second shield being composed of a closed circuit resistive coating, said coatings having suflicient resistive values to avoid the effects of a short-circuited turn overloading said transformer, and to avoid a resistance so high as to impair the shielding effect of said shields,

a third insulator around said second shield, and

saitd second winding being wound on said third insula- 5 '7. A device as in claim 6 wherein 2,652,521 said resistive coating is a resistive paint, and 2,724,108 said insulators are composed of a plastic tape wrap- 3,032,729 ping. 3,041,561 5 3,149,296

References Cited by the Examiner UNITED STATES PATENTS 1,320,980 11/1919 Bowman 336229 X 6 Westphal 336-84 Hayes et a1. 336-84 X Fluegal 336-84 Hannon et a1. 336-84 Cox 336-84 LEWIS H. MYERS, Primary Examiner.

ROBERT K. SCHAEFER, Examiner.

1,837,903 12/1931 Goldschmidt 336 84 X 10 W. M. ASBURY, T. J. KOZMA, Assistant Examiners. 

1. IN A TRANSFORMER HAVING AT LEAST A PRIMARY WINDING AND A SECONDARY WINDING, AND INSULATION ON EACH OF SAID WINDINGS, THE IMPROVEMENT COMPRISING A SHIELD BETWEEN SAID WINDINGS AND COMPLETELY ENVELOPING ONE OF SAID WINDINGS, SAID SHIELD BEING CONSTRUCTED OF A CLOSED CIRCUIT RESISTIVE COATING, SAID 